[FEATURE] git-gui: Staging path(s) should re-select a new path in "Unstaged Changes"

When a path is selected in "Unstaged Changes", it can be staged (obviously). When it is staged, the path goes to the "Staged Changes" list, and no (new) path is selected. I propose that this action should re-select a new path, from the list of paths in "Unstaged Changes".

Steps to reproduce (in git-gui):
1. Open git-gui in a repository which has two or more uncommited, changed files.
2. Select a path in the list of "Unstaged Changes"
3. Click CTRL/CMD+T to stage the file.
4. Observe how no (new) path is selected, forcing you to re-select
another path if you want to stage it.